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How much energy is "wasted" in an aeroplane's descent?
In descent, an aircraft must lose both potential energy (in order to get down to the ground) and kinetic energy (in order to land at a safe speed).
Weight and drag can take care of that. An unpowered plane will eventually slow down and sink. However, sometimes it won't happen fast enough without additional control input, and the pilot must act to make the plane less aerodynamically efficient, and dispose of some surplus energy that would otherwise be retained longer.
If a plane completed its descent from top to bottom without using the control surfaces to:

dump excess lift
bleed off excess speed along the way

until actually landing, that would mean no waste of this energy in descent: all of it was used productively to get the aircraft to its destination, and none of it needed to be thrown away to get the plane low enough and slow enough fast enough.
In a particular descent, would it be possible to calculate or estimate how much of that energy in practice does in fact get "wasted", i.e. needs to be deliberately thrown away?

I think the question is inherently somewhat problematic I guess, in this conception that certain part of the potential energy loss is "productively used" and a certain part of it is not.  See comments below my answer.

The plane has an altitude and some hundreds of km/hr (potential and kinetic energy) but the goal is to stop the plane so that passengers can come out safely. You want to dump all this energy. I agree that the question is problematic... You probably want to pose the question differently..

To calculate this energy you simply need to know the mass, the altitude and the velocity of the plane. If you wanted to calculate how far the plane would 'glide' (converting altitude into kin. energy without airbreaks) you need the aero coefficients and simple flight mechanics equations. So for a specific answer you need to specify the type of plane. Otherwise the question is overly general.

I guess "deliberately thrown away" could mean any energy lost due to the deliberate increase in drag caused by deploying the flaps and spoilers.  Keeping in mind though that as we slow down below high-speed cruise speed we reap some benefits in efficiency.  So compare the actual energy change, to the energy change that we'd see if we maintained the cleanest configuration for the same time period, and brought the plane to the minimum sink rate airspeed as efficiently as possible and then held that airspeed for the same period of time?  Something like that.  There could be other variations.

@quietflyer Exactly, as the question says, "using the control surfaces".

Is there some reason this question has attracted negative votes? It seems pretty clear to me, and so far three different people have been able to provide answers that address the question being asked and help answer it in intelligent insightful ways.

Should we also count as "wasted" the energy spent in cruising at high speed for a certain time interval before the steep descent was started, compared to the energy that would have been saved by starting a  clean, efficient, low-power descent at the max glide airspeed at that point?

In the latter case also using the most efficient way possible (possibly even including a climb to gain altitude) to transition from the high-speed cruise to the max-glide airspeed?

Lots to consider here.  I don't think it's a bad question as it has provoked some interesting conversation but I'm not sure that the question is as defined as it could be.

A meaningful numeric value is going to be hard to calculate, because so much depends on the aircraft you're working with.  In practice, if I can pull off power at the calculated Top of Descent point, descend, meet any ATC crossing restrictions, fly the approach, configure as late as possible, and push up the power right at the point where I'm required to be stabilized for the approach, then I've flown an efficient approach & wasted none of the energy that I started with.
If I have to add power sooner than that, I started down too soon (or wasted energy in some way); if I have to deploy drag (which includes configuring early), then I started down too late & so I have excess energy that I need to dissipate.
I suppose "amount of time speedbrakes were up" would be one way to score how much energy had to be wasted; add to that how long flaps & gear were out before they were "needed" (given the requirement to be stabilized & configured at a given point in the approach).
Here's the complication, though.  If I see early on that I have too much energy, I can push the nose down & gain speed, which increases the parasite drag of the aircraft -- thus dissipating energy.  If you have enough time at the higher speed (not just push the nose down, gain speed, pull the nose up, lose speed -- that doesn't accomplish very much), then you can get rid of excess energy without having to deploy speedbrakes or configure early.  And the reverse, if my standard descent is at 280 knots (above 10,000', obviously) but I slow up to 250 knots early enough, I can "gain" some energy, since I'm losing less with less parasite drag.  How you could account for all of that in a "score" is beyond me.
In the ideal case, I can descend to glideslope intercept altitude, level off there, let the speed decay, then at the point when I need flaps to fly any slower, I extend the first increment of flaps, let the speed keep dropping off, start down the glideslope (speed decreases more slowly or if the aircraft is light enough, not at all), at about 500' above the "stabilized approach" gate I'll lower the landing gear & extend more flaps, so that as I approach that gate I'm extending the last increment of flaps, my speed reaches VTarget, and I add power.  No extra time with power above idle, a fuel-efficient approach.
ATC may not let me do that, though; they may keep me fast until close to the runway, in which case I'm extending flaps & gear closer to their maximum speeds so as to dissipate more energy more quickly and still be configured & stabilized at the appropriate point.  (The 737 dissipates speed pretty quickly fully configured at idle power, even descending on the glideslope.)  So did I "waste" some energy in that scenario?  Yes.  But keeping ATC happy so they can get efficient runway utilization is a worthy goal as well.

I think this answer touches on an important point: while it's possible that you need to waste some energy because you're high or fast, the alternative may be that you need to expend some energy if you're low or slow by adding power at low altitude (which is much less efficient). And if you take into account that lower runway utilisation means more airplanes have to take a detour or hold, it seems that "energy wasted" is a function of the airport environment rather than a single number.

In a particular descent, would it be possible to calculate or estimate
  how much of that energy in practice does in fact get "wasted", i.e.
  needs to be deliberately thrown away?

The same question, but from a different perspective, is "how much energy margin is typically retained during an approach?" Having flown gliders, this is a near-and-dear question to my heart.
Assume you are going to deadstick the landing. From a point in the sky at a certain height (potential energy) you can come up with an altitude versus distance chart to compute how far away an airport can be and basically roll out at the end of the arrival runway having not used brakes and not moving. If assuming standard atmosphere, you can derived a closed form equation. The potential energy at the start is the bare minimum energy needed to get to that point. (There is some kinetic at the start too, but gets dwarfed by the potential.)
http://www.dept.aoe.vt.edu/~lutze/AOE3104/glidingflight.pdf
Your speed to fly would be L/D max speed.
To figure out the excess energy, just look at your start altitude, and then the finish altitude for the actual distance to fly. The energy at that finish altitude is the excess energy "wasted" or to be considered as margin.
I used to do this. On an OCF (out of control) flight, we'd start our last inverted spin at 27,000 ft, in our working area in South Texas. My goal was to pull the power to idle at the start of the spin, and if possible not touch it again all the way to parked in the chocks (very runway dependent). The working area was about 25 miles away. We'd come out of the spin, orient, and fly at max range-ish profile to the field, get a sense for the pattern, and if open, basically dive bomb to a tight initial, enter the break at 400+ kts, break at the numbers, fly the circling approach, and if on the right runway, the turnout at the end was right by the parking, and if they were almost full, the last spots were by the turnout, with a wee bit of downhill to the chocks. All at idle throttles. Woot!

First one may consider a glider at Vbg.  This is the most efficient use of its potential energy.  "Wastefullness" would be any departure from Vbg and could be quantitatively determined from flight data and drag curve graphs.
For powered aircraft, the same determination could be made from departure from both aerodynamic and engine/prop peak efficiencies.
Also, consideration of departure from direct flight due to holding patterns or other traffic control could play a role in long term efficiencies for a large fleet of aircraft.
However, other factors, such as safety or expediency limit the applications of the concept.
A flight from New York to Los Angeles will not slow down to save fuel.  New York to Atlanta,
may not be a bad idea.

I'm only concerned about the descent phase of flight in this question.

@Daniele Procida Would that be wasted as far as diversions or holding, or would it be related to the way the aircraft was flown?  There are also speed "limits" based on altitude for airliners, and changes in drag when they re-configure. If you can be more specific about the application, it might help.

I'm just interested in the aeroplane as a flying object subject to physical laws.

@Daniele Procida This is a very good subject as they really try to BUILD aircraft to be fuel efficient, yet do not FLY them that way.  Why not include climb and cruise as well?

I agree that the question of fuel efficiency is important, it's just not what I'm asking about. It's a question about physics in aviation.

Sure.
Let's take the simple case where mass is assumed to stay constant during the descent.
The formula for kinetic energy is KE=(1/2) * m * (v^2), where m is mass and v is velocity.
Compare the kinetic energy at start and end of descent, using True airspeed.  If True airspeed is constant, Kinetic energy is constant.
Now compare the (gravitational) potential energy at start and end of descent.  Potential energy = mgh, where h is elevation and g is the gravitational constant and m is mass.
The energy lost is the decrease in potential energy, minus any gain in kinetic energy.  Or the decrease in potential energy, plus any loss of kinetic energy.
If airspeed is constant throughout the descent, we are losing ALL of the potential energy we had at the start of the descent-- NONE of it is being converted to kinetic energy, or to any form of stored energy.
Too bad we aren't coming down by using a prop as a windmill to power up a battery as well as create drag, rather than by opening spoilers and such.

Alright, expanding this answer to consider comparing the energy lost in the actual descent to touchdown, to the energy lost in in some representative "ideal" maneuver.
What is the "ideal" maneuver?
Ideal Option 1)  -- at the same point in time that the descent would be normally be initiated, we instead smoothly pull back on the yoke to transition to the angle-of-attack that gives the max ratio of lift/ drag (unless this would arc the flight path up beyond vertical unto the second quarter of a loop, then we have to use a slower rate of increase of angle-of-attack to pull less "G".)  Anyway we end up doing a (possibly rather steep) "zoom" climb to gain altitude and lose airspeed and then we let the flight path arc back down into a steady-state glide at the angle-of-attack (and airspeed) that gives the max ratio of lift/ drag.  As we overfly the point where we'd touch down with the real-world method, we note our altitude and airspeed; this gives us a basis to compare the actual energy lost in the real-world method with the energy lost by the "ideal" maneuver.
2) Ideal Option 2-- we roll back time to the point where we can glide all the way to touchdown with the plane in a clean configuration, at the speed that gives the flattest glide, after initiating a highly efficient transition to the best-glide speed such as that described in Ideal Option 1.  At the moment of touchdown, compare the fuel in the tanks at the moment of touchdown with this Ideal Option, to the fuel in the tanks with the standard method of a longer cruise period followed by a steeper shorter descent. Also consider the difference in airspeed at the instant of touchdown.  Now we have a basis to say how much energy is "wasted" with the standard method (but how do we evaluate the energy stored in the fuel?  Do we consider ALL the energy stored in the chemical bonds, or just the energy that the engine would actually be able to extract?)
3) Ideal Option 3-- instead of descending we just maintain cruise speed and altitude so we lose neither kinetic energy nor potential energy.  So no energy at all is expended with this ideal option.  Well, that's a little silly and shows that we really do need to take into account the potential chemical energy we're taking out of the fuel tanks, both with the actual method and with the ideal method.
Since we're now considering the energy stored in the fuel, we now ought to revisit the ideal option 1.  How much power should we carry?  Just enough that the engines create zero thrust-- keep the fans turning under power so we're not using them as giant windmills?  Maybe just a little less than that?  Surely we don't want to chop the fuel flow to zero do we?  Then the fans would make way too much drag.
On the other hand if we are considering, say, the energy that could be released by the fuel in a nuclear fusion reaction, then we better leave it all in the tanks for the "ideal" descent option, as it is too precious to waste any.
While we're at it we should really note that with the real method-- and arguably with the ideal methods too -- all the kinetic energy that we're carrying at touchdown is wasted-- for the real-world method at least I guess we really need to measure all the way to the point where the plane comes to a stop with zero energy.  Also consider all the fuel wasted as we apply reverse thrust.  None of this comes into play for the "ideal" methods; we get to use the most efficient method of flight possible to keep the plane moving through the air till we are overflying -- the touchdown point?  The point on the runway where the plane would come to a stop in the real world?  The gate at the terminal?  With the no-landing ideal methods such as #1 and #3, as we're about to overfly the gate at the terminal, high above it, do we get to ease the yoke further back and bleed off airspeed all the way to stall speed, helping us to maintain altitude for a while without burning (much) fuel, or even to gain some altitude?  Sure, why not.
Lots of different ways to set up this problem.

Kinetic & potential energy for aircraft don't work the same as KE & PE in marbles rolling down ramps and across tables in the Physics lab.  A ball rolling across a level table can be said to "lose" no KE or PE; an aircraft can't do that.  Some sort of energy (chemical, as thrust, or KE as slowing down, or PE as drifting down) MUST be expended to overcome induced & parasite drag and keep flying.  Doesn't mean your PE is "wasted"; it's being put to use.  If you deploy lots of drag in order to descend quickly, then you're wasting your PE, and that is what the OP is asking about.

Hmmm, I don't think I agree.  A glider with a very high L/D ratio loses very little potential energy per mile travelled.  An airliner coming down w/ flaps full down and spoilers full open and those big fanjets hanging out in the wind has a much poorer L/D ratio and a much steeper glide angle and loses a great deal of potential energy per mile travelled.  Guess I'm not seeing the problem with my answer.   PE lost is PE lost.  If it's not being converted to KE or stored energy than it's lost.  You seem to be using "wasted" in some kind of subjective sense.

Your answer is right in the physics classroom, but look at how the OP defines wasted, or "no wasted energy."  That's where I think this answer doesn't address the question as stated.

@RalphJ maybe you want to define "wasted" related to the total energy ACTUALLY lost in the descent compared to the total energy lost in the MOST EFFICIENT POSSIBLE descent for that particular aircraft?  I don't think that's implicit in the question though.  And then we have to decide if we want to define "most efficient" as minimum energy lost per unit horizontal distance travelled, or per unit time, or per unit vertical distance descended, or what-- anyway I don't see that as being implicit in the original question.

OK I see he did put some constraints like that in the question-- I think the question is inherently somewhat problematic I guess, in this conception that certain part of the potential energy loss is used "productively" and a certain part of it is not.

She, that is.  The O.P.

@RalphJ is correct about what I am asking about. "Wasted" or "thrown away" energy is the energy we deliberately dispose of so that the plane descends and decelerates more quickly than it would otherwise.


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What is typical thrust to braking force ratio for a passenger airliner?
Is the ratio fairly constant across aircraft types, sizes and powerplants?
I'd like to ignore the reverse thrust component, since that's not available at the same time as forward thrust.

Can you clarify what you mean by "thrust braking force ratio" if it's not the ratio of max forward thrust to max reverse thrust?

It depends a lot on the weight. Thrust is independent on weight, but break force is almost linear to weight.

If we assume that the braking system is designed to leave no unused braking capacity, the maximum braking deceleration is determined by the friction coefficient of the tyres. This NASA report gives a friction coefficient of around 0.5, for dry concrete with a functioning anti-skid system. This results in maximum braking deceleration of 0.5 g, around 5 $m/s^2$
A typical maximum thrust acceleration would occur for a typical airliner, at TO thrust, and medium range TO weight. Let's take typical numbers for an A320:

A320: TO thrust = 2 * 120 = 240 kN, TO weight = 70,000 kg, a = F/m = 240/70 = 3.5 $m/s^2$
B777: TO thrust = 2 * 500 = 1000 kN, weight = 300,000 kg, a = 1000/300 = 3.3 $m/s^2$
Fokker 100: TO thrust = 2 * 67 = 134 kN, weight = 40,000 kg, a = 134/40 = 3.35 $m/s^2$

So across the board, the braking-to-thrust force ratio is around 5/3.3 = 1.5.

That's consistent, but considerably lower than I expected.

Yes, I remember learning a long time ago that hard braking was actually close to 1g but could not find a reference to it.

Does it mean that at a dry concrete, a plane with TO thrust and full brakes won't ever move?

@yo' It also needs to be at TO load. The ratio could easily reverse if the plane is not heavily loaded.

@user3528438 Ah right! The max friction force is of course proportional to weight! Thanks for the hint!

Information indicates that to calculate a disk braking force, you also need to know the coefficient of friction of the brake pads, the force the pads apply, how many there are per wheel, and the radius at which they act on the disk; for each wheel. The size of the tyre contact patch I expect should also be a factor. This make engineering sense that these factors all effect brake performance. What do you think, or did I misunderstand something? One might be able use v^2 = u^2 + 2as for the landing roll, but this assume linear deceleration, and the plane comes to a complete stop.

@Richard we're doing an estimate here, not unlike what is done in the pre-design phase of an aircraft. What we don't want to do at this stage is lose ourselves into details before we've gained a ROM number. Looking at this from a helicopter view, it makes sense if the brake system is designed such that the friction between wheel and runway is the limiting factor, otherwise there is brake capacity left that cannot be effected.

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What is the explanation of the apparent staining inside this airliner window [photo]?

On a recent flight, I noticed a pattern of staining or dust particles inside the window, on the plastic reveal within the cavity.
It looked as though a long-term and consistent pressure differential between the cabin and the cavity had drawn a stream of air into the cavity, resulting in what looked like a sooty stain.
And even more oddly, as you can see in the picture, right opposite the vent, the mark is very concentrated, as though formed by a high-pressure stream - which I presume isn't actually the case.

Related: https://aviation.stackexchange.com/questions/2454/why-is-there-a-hole-in-one-of-the-layer-of-an-airplanes-windows/2455#2455

@mins The mark in the photo is not on the external glass but on the plastic surround or reveal inside. I find it hard to credit that cabin pressure could increase so rapidly as to produce such a tightly-focused stream of air onto the surround. Look at the diameter of the black centre of the stain.

It was just an ordinary 737 or E170, no fancy electronic windows.

I believe the stain is caused by air going through the hole relatively fast due to compression of the cabin. Assuming the window is 40x40 cm large, and there is 10 cm between the panels, the volume of air in it is 16.000.000 mm3. About 30% of this air comes out due to the lower pressure at altitude, that is 4.800.000 mm3. Assuming it takes 600 seconds (10 min) to pressurize the space again, the flow will be 800 mm3/s. Assuming the hole has a diameter of 2 mm, it has an area of 3.14 mm2. The average flow velocity is then 254 mm/s. It looks like the spot is only 20 mm away from the hole, so it would take a dirt particle less than 0,1 second to get to the spot from the hole. I think that is not much time to deflect, hence the concentration in the middle.

@mins: Thanks! I suppose I should have been a bit more clear there. I mean that 30% of the air between the two window planes comes out of that space, and goes in to the cabin. This is because the cabin pressure is kept at approximately 70% of the pressure at sea level, and the space between the windows will have the same pressure. Then according to the ideal gas law, assuming constant temperature, if the pressure is reduced by 30%, the volume of the gas increases by 30%. Since the space does not get any bigger, 30% of the gas will move to the cabin.

Ah indeed, btw an interesting answer.

This is a brilliant answer, and an excellent example of how calculation can arrive at a truth that confounds intuition. Two questions however: A) Is the plastic pane sealed? They often feel quite rattly; if air can enter the void from around the edges of the pane, then the sums would be quite different. And B) there's considerably less than 10cm between the panels, it's more like 4 or 5cm. Do the sums still work?

@DanieleProcida: Thanks for the complement, I found it a very interesting question. About the questions; A) If it is not sealed, none of it will hold. Even a 0,2 mm gap, which is not even visible from this angle, would make an opening about 100 times bigger than the hole. 99% of the air would come in through the sides and the speed would be 100 times lower. That won't leave much of a stain.  I believe it is sealed though. Left of the hole you can see the seal alignment tab, also shown in the first figure Peter Kampf referred to. Without seal it would rattle like crazy due to engine vibration.

Continued: B) It looks like at least 10 cm to me, I was even thinking about going for 15 cm. But with 5 cm the speed would be half as high, 127 mm/s, which is still a lot. Imagine scaling the whole thing to human size, a 2 meter hole. The speed would then be equivalent to 127 m/s, or 457 km/h. If a car were to pass close by at that speed it would be gone already before you even realize what just happened.

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To what extent does pressurisation increase the rigidity of an airframe?
A vessel such as a balloon or the rubber air-cell of an inflatable boat will become more rigid when pressurised.
The same must happen to the fuselage of a pressurised aircraft - but to what extent?
Unlike balloons and inflatable boats, aircraft are not flaccid when unpressurised...
I imagine that the effect must be so tiny as to be not just utterly negligible, but literally impossible to measure; is it possible however to calculate theoretically (and if so, of what order is it likely to be)?

It's linear, so you can do the calculation as if it were a long balloon and that's the added rigidity.

Balloons become more rigid because they have high plasticity. Airframes are already rigid, increasing the air pressure inside (as relative to external pressure) shouldn't increase it's rigidity at all, it narrows the gap between the material plasticity and it's yield point. The more times this happens, the worse it gets, like bending metal back and forth until it snaps. It's not impossible to measure, it's quite easy using strain gauges to measure surface deflection, I've done it on ice breaker ship hulls.

I flew with a C5 pilot who told me that they had to observe extra operational limitations when unpressurized (ramp down, doors open, or just inop) because the pressurization actually provided rigidity.

This is especially relevant for rockets. If they are not pressurised, they cannot be transported.

@PeterKämpf: Depends on the rocket.

Pressurisation indeed increases rigidity of the structure. Air pressure expands the fuselage, and applies a tension stress uniformly throughout the fuselage skin.
When in the air, the aircraft is suspended by the wings and garvity attempts to bend the fuselage downwards. The failure mechanism of bending a tube is buckling: the top side has tension stress, the bottom side compression stress, and this is the side that buckles. Internal air pressure helps in two ways:

The bottom side has pre-tension stress applied by the air pressure. The gravity bending is now resisted by a reduction in tension stress, instead of an increase in compression stress. This results in less deflection difference between upper and lower surface - higher stiffness.
The fuselage expands as a result of the overpressure, increasing the distance between upper and lower surface and therefore reducing reaction stresses from bending.

We don't want to compress sheet metal, it buckles pretty quickly. However if air presses against it, that helps in preventing that particular failure mode.

Disclaimer: I am not a pilot, nor do I have anything with planes. I am a mechanical engineering student, so I do know quite a bit about the strength of materials.
What you should know is that whenever something is filled with air, that air is trying to achieve a state in which it has the lowest pressure, meaning it is trying to increase in volume. This is the reason balloons are roughly round, because a sphere has the largest volume for the lowest surface area. The same pretty much applies to planes. What generally happens when you bend a tube (and a fuselage is in essence a big tube) is that two sides are getting closer, making it easier to bend because the second moment of area decreases. This does however mean that the hull becomes less round, thereby decreasing the volume. The air inside will try to prevent this volume change,  which makes it harder to bend the tube, which means the stiffness increases. Note however that more stiffness does not necessarilly mean it is also stronger.

This is a good explanation of why the aircraft is stiffer - but doesn't answer the question of how much. A quick back-of-the-envelope to give an order-of-magnitude estimate should be easy for a mechanical engineer :-)

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Why did the pilot flying pull back in the moments after the auto-pilot disconnected on AF447?
In the AF447 disaster, one of the elements that contributed to the crash was an "excessive" nose-up input shortly after the disconnection of the auto-pilot.
This caused the aeroplane to climb and lose airspeed, precipitating the ultimately fatal stall.

Why did the pilot flying pull back (excessively) on his stick? Did the investigation find probable causes?

He was at the same time correcting for turbulence-induced roll. Would a nose-up input in such circumstances be appropriate?

If so, how easy is it for a pilot to make an excessive input of that kind without realizing it, and what would normally prevent that from happening?
If not, is there any explanation for why he might have made it?

Most of the crew's actions in the incident are comprehensible, even if they were incorrect responses. They make sense in the context of the situation, and it's possible to have a fairly clear picture why they made the judgements and decisions they did.
Is there a good explanation for that initial nose-up input, that makes sense of it too in the context?

I'm voting to close this question as off-topic because accident speculation is explicitly called out as off-topic in Help/On-topic

We have a few questions about AF447 already; this one might have some relevant info. And the official report is available, that has all the objective data.

He didn't recognize the stall, the captain did but by the time he realized that the control inputs were wrong it was too late. This accident resulted in more stall training and hand flying for pilots when automation fails.

Daniele, have you read the report issued by the French accident investigators?  It's pretty thorough (BEA).  If that body after an exhaustive investigation, could not come up with the why, who do you think can?  The "why" being nailed down requires something like mind reading.  If you have not read the report, I suggest you do that.  Once you've read it, consider editing your question to reflect what you learn.

@KorvinStarmast it addresses why he held that attitude for an extended period of time but doesn't really say why he pulled back at the beginning. the aircraft didn't stall until later after airspeed bled off and the nose was held up.

How are questions like Would a nose-up input in such circumstances be appropriate?,  how easy is it for a pilot to make an excessive input of that kind without realising it, and what would normally prevent that from happening? or is there any explanation for xxx? "opinion-based"? I am not interested in guesses as to the contents of his mind; I am asking about piloting responses to conditions. Anyone who imagines those are a matter of opinion doesn't understand aviation. Or perhaps, questions. If the answer is that his initial action remains unexplained, that's also a possible answer.

@RonBeyer Please read the question. I'm not asking about how the crew failed to recognise the stall, or anything else about the incident. I am asking about why Bonin made the initial nose-up input as a response to the conditions (turbulence, reduced speed, roll and over-correction). Was that an appropriate piloting response? Is it one that is easy for a pilot to misjudge? I can repeat the questions in this comment, but they were in the question to begin with...

@DanieleProcida you have to consider that the title influences the way the post is read and received. I will try to adjust the title, but please review my edit afterwards. Also, asking "why person did X" is opinion based, we are not in their brains.

Closed as opinion based? There is an accident report which also addresses this issue, hardly opinion based I reckon. It's a different matter if there is no accident investigation report yet, as the help centre (which needs to be re-written anyway to reflect what the site morphed into!) clearly indicates. Have voted to re-open.

@Federico "Why" can address objective causes and reasons (as well as personal motivations, states of mind, etc).

@DanieleProcida Have you read the BEA report or not?

@KorvinStarmast I do not understand the challenging tone of your repeated question.

Daniele, I believe that you will ask a better question if you read that (very well written) report.  When you edited the question back to "why" it sent a signal that you are not taking these comments on board.  I will again strongly recommend that you read the report.  It is well worth the time.  There isn't a simple answer.

@KorvinStarmast The reason I removed the change to "probably causes" from the title is because it was simply wrong. None of my question was about causal factors, other edits to it notwithstanding. If there were a simple answer, I'd expect it to be generally understood by now. You're making rather a lot of assumptions.

Daniele please read pages 43-45 of the link I provided.

Daniele, those are references to the pdf file pages, the report pages as numbered in the report are 41-44.

First Officer Bonin's actions are almost a textbook example of the startle effect.
Simply put, he was shocked by the sudden disconnect of the autopilot that he mentally froze and couldn't think straight. He appears to already have been in a state of anxiety due to the St Elmos fire immediately beforehand, and the thunderstorm in the vicinity. It is a very interesting but sad human factors discussion.
A good explanation can be found here.

@KorvinStarmast Edited to make it clear that I was referring to Bonin. Physically he did not freeze, but mentally he was stuck on his initial reflex, and could not take a step back to figure out what was happening.

Yeah, I see the point you are making.  Also fits better with your link.

@Ben I don't believe this is entirely correct. Although the startle effect is certainly implicated in the case, far from freezing, Bonin appeared to make a number of immediate corrections. Some were appropriate in kind if not degree (e.g. he overcorrected for roll) but the initial nose-up input of those first few seconds is harder to fathom.


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What causes airliner tyres to burst as often as they do?
Tyres burst - not just fail - remarkably often. The Aviation Herald lists 26 tyre-related incidents this year.
I'd expect fast landings or overweight landings and heavy braking to be implicated, but often, tyres burst during the take-off roll:

Nordica CRJ9 at Kiev and Tallinn on Jun 4th 2019, burst tyre
American B738 at Kingston and Miami on May 7th 2019, tyre damage on departure
American B738 at Vancouver on Apr 19th 2019

Some other failures also appear very regularly (cabin pressurisation, flaps, landing gear). However, these are of complex systems that have many interacting parts, and they also exhibit multiple ways of failing.
Car tyres, which are maintained much less well than aircraft tyres and travel far greater distances over much harsher surfaces, very rarely burst, even when they fail.
Why do the tyres of these large aeroplanes burst rather than fail in other ways? What is it about the way that aircraft tyres are made, or the conditions that they are subjected to, that cause them to burst as often as they do?

When you say "so often," that seems to perhaps lack context.  If things from Kiev to Kingston are being considered, that's probably on the order of 26 occurrences out of several million flights. Which perhaps isn't quite as often as it first seems.

It's as often as it is, which is what I am asking about...

Car tire likely blow out more often but the consequences are not  newsworthy ( except to the driver) .

@blacksmith37 Car tyres almost never burst. They deflate, lose pressure, become unseated, wear out, delaminate; are torn, punctured, cut, shredded. I have not once heard of a car tyre bursting.

@DanieleProcida in everyday usage, sure, they don't burst very often. But then inner-city driving is much kinder to tyres than what plane tyres go through on take-off/landing. You'll see a couple of tyres blow up over the course of a typical F1 season, though.

I have had 2 burst , like a firecracker,; one , a Goodrich steel radial , the other a no-name bias ply.

You may rephrase your question so that it is no longer opinion-based and answers still provide clue you are looking for.  Something like "What are the cause of tires burst?" and then emphasize the fact that tires burst in the specified occurrences, not just deflated of fail.

@DanieleProcida: On the other hand, truck tyres burst semi-routinely; that's where the pieces of disintegrated tyre you see from time to time by the sides of interstates come from.

Car tires aren't really comparable.
Car tires:

make a lot of miles, but in mostly benign circumstances. Low speeds mean little heating (and slow changes in temperature).
failures are often caused by debris in the road rather than structural failure of the tire itself. (Although on trucks, structural failure is more common)
must be replaced when the tire profile reaches a minimum depth, which means you can't keep using the tire until structural failure happens.
are generally thrown away when the profile reaches minimum depth, because car tires are cheap enough that retreading is not necessary

Airplane tires:

rapidly go through temperature cycles (in-flight: -50 ºC, and on touchdown they rapidly heat up to several hundred ºC)
are kept at much higher pressures (13 bar versus 2 bar for car tires), so if there's a failure it's more likely to be catastrophic.
need to run at high speeds (300 km/h and more)
carry a lot of weight (up to 38 tons, vs. 500 kg for a car tire and 5 tons for a truck tire), which explains why they need such high pressures.
have to be lightweight
are often refurbished

Thanks, that point about pressures explains a great deal.

You missed "cope with extremely high changes in speed at touchdown"

that's mostly covered by 'rapid temperature cycles', as the acceleration in itself won't damage the tire.

@Hobbes, it's not just the temperature, but also the scrubbing as the non-rotating wheel suddenly touches the concrete moving about 300 km/h.

It really isn't that common in the big scheme of things, but there may be a factor that makes them more common than they might otherwise be.  A lot of airlines retread their tires because they go through them so fast (try the life of a tire, going from 0 to 140 in half a second, then having to deal with producing extreme braking friction, 6 times a day, and see how long you last lol).
You can recap airline tires indefinitley as long as the basic carcass is within certain serviceability limits.  An airline will send their worn tires to Goodyear or Michelin to have them basically remanufactured and they are theoretically as good as a new one, but in reality probably not quite (but good enough).
If an airline uses new tires all the time, they might expect to have fewer tire disintegrations, but the costs would be astronomical and I would say the majority of operators recap them until the carcass has to be scrapped.

Please don't reference comments in an answer. Comments are prone to being removed making this answer confusing. Your answer should stand alone.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
What would the altitude profile for a typical airline flight look like?
It's hard to appreciate an aircraft's rates of change in altitude, in particular with respect to its travel through the mass of air, rather than over the ground, since the latter is affected significantly by wind-speed (the distance an aircraft travels with respect to the ground is highly variable, compared to the distance it travels with respect to the air around it).
Then, when climb profiles are plotted, it's always on a graph with a hugely compressed horizontal axis (naturally, because it's hard to discern much information when one axis is a thousand times longer than the other).
Finally, when climb/descent rates are given they are usually in vertical distance/minutes, which is useful from an aviation point of view, but hard to compare with a vertical distance/horizontal distance measurement, or for example the familiar gradient figure that would be used to describe the slope of a road.
So, assuming absolutely still air throughout a flight, what might the altitude profile as plotted against distance look like, for a typical airliner on a flight long enough to be at cruising altitude for a few hours?
And, what sort of gradients would be represented for its climb after take-off and its descent towards its destination?

I can probably add some info later, but in the meantime - why don’t you convert one of the climb profiles you have into a diagram with a common axis scale for the visualisation you would like to see?

If I have to assume "absolutely still air throughout a flight" the plot given by any online flight tracker answers your question. The problem in what you ask is that unless you are equipped in recording the wind conditions throughout a flight, you will never be able to reconstruct what you seek.

@Federico Since aircraft are equipped with both altimeters and airspeed indicators, this information must exist. I'm pretty sure pilots have an idea of the figures, for example. And clearly the information I'm after will not be available by looking at online trackers, which are a complete red herring here. I'm not interested in reconstructing information from some particular set of data, I'm asking about what a typical example might look like.

Daniele, may I suggest you pass by in chat? I am not sure I follow you, and if the discussion drags on it is better to have it in chat.

I have written here what I do not understand Please join

It is somewhat unclear what you want to achieve.
If you want to know how it looks like, you can search for some papers like this. To keep it short, altitude vs distance curve is pretty much similar to altitude vs time, but climb and descent segments are more curved with larger second derivatives for climb and descend. For details please read the paper.
If you are interested in some analysis, you need two components: flight data itself and data analysis software.
If you are looking for professional flight data analysis systems, for example, flightDataServices, generally they are not provided for free, but even if you have an access, you still need flight data for analysis. It can be more difficult to find it. If that is what you are looking for, just google "Flight Analysis Software" and "Flight Data".
You can do that analysis by yourself though, with a little less accuracy than a professional system. The closest option for getting an accurate flight data is FlightAware.com. They do not provide altitude vs distance out-of-the-box, but they provide plenty of data for further analysis. For example, this flight. You can open track log and see altitude/speed vs time (which is not you are looking for, but useful for a 'big picture'), and in the log itself you can find latitude, longitude, speed and rate of climb for some points. The points are provided with a less-than-a-minute frequency, which is pretty precise. It is not so hard to export them in any spreadsheet or CSV and generate whatever you are interested in: a pretty common data visualization task. It can be easily done with Matlab/Excel/LibreOffice. For example, altitude vs distance travelled, course vs time, rate of climb vs time, rate of climb vs distance etc.
If you are looking for data visualization software specified for flight analysis, I am not sure that something exists that fits all requests. Especially an online tool. The reasons are: flightaware provides both data and the most common plots (i. e. alt vs time and speed vs time); and if you already have data, it's pretty easy to plot whatever you want using Matlab/R/Python/Excel.

Well, it doesn't include data for altitude/distance, which is what I'm concerned with, so no, unfortunately it doesn't provide the most basic thing I am asking about. It provides data instead for altitude/time (which as the question points out  is readily available but not what I am looking for).

It provides latitude and longitude, and speed. You can copy that data to any tool (even Excel) and plot altitude/distance.

That would a) be quite a lot of work in itself, for something that I hoping to find exists already b) would be very hard to do accurately taking into account the fact that aircraft typically make multiple turns particularly in climb and descent phases, and c) fails to take into account that it's distance through air, not distance over ground, that the question is concerned with.

Yes, it is a little bit of work in itself, copy, paste to any spreadsheet and make 4-5 mouse clicks after that to create a diagram. And about all other points... You have a horizontal and vertical profiles with latitude and longitude in a track log. What distance through air are you talking about? Lat-log are ground references! All turns of airliners are usually very shallow, and positions in a track log are given at least once a minute. Pretty accurate for computation.

Please read the question again. I know you are trying to be helpful, but I spent quite a bit of time formulating the question to specify what, exactly, I was trying to find out. Possibly I didn't express it as clearly as I'd hoped, and thank you for trying to answer, but the information you keep trying to tell me about is not the information I am seeking. I don't think you have understood the question.

And I genuinely do appreciate your answer - it's just that it doesn't answer my question.

OK, I updated my answer

Here is a flight profile graph that shows altitude plotted against time, as you mention is often the case, but it also includes airspeed which gives distance traveled through an air mass as you require. Airspeed can be used to stretch/squeeze the x axis of the altitude diagram to show distance through the air instead of time, and this changes little once the aircraft is above 20kft.
If you assume the x axis is ~400kt, the climb phase at ~200kt would squeeze it by half.

Are you sure this graph is airspeed and not ground speed?

What would the profile “look like”?  Well, if you were to graph it, it would be very, very long and skinny...  (there is a reason why vertical scales are typically exaggerated)
That might sound snarky, and I know you think you put a lot of thought into the careful wording of your question, but it is very broad as stated:  What are you really looking for in an answer, the general appearance of the profile when plotted with vertical and horizontal scales equal?  The climb or descent gradients expressed in degrees?  Or rise/run slope?  What units of measure would you like the answer to be in?
And what is a “typical” flight?  (2 hours of cruise, or 12 hours of cruise?!)
I know you said you don't want to crunch actual data, but I think the suggestions here are sound if you are looking for typical.  Any effects of wind will average themselves out as you increase your sample size.  And there are other variables that have as much or more effect on the climb gradient.
However, if you are looking for a ballpark answer, here is my shot at giving you some additional perspective:
Climb - Airliners don’t typically target a vertical speed in the climb because doing so will cause airspeed to gradually decay.  Instead, a climb airspeed is set, making the vertical speed a continuously changing variable.  And that vertical speed will vary depending on aircraft type, the load, and whether a fuel efficient cruise climb is chosen, or a higher performance climb rate for terrain clearance.  (so again, no such thing as “typical”…)
Cruise - Get a long piece of graph paper because "the altitude profile plotted against distance" for 4 hours at 300 knots at 30,000 feet will be 243 horizontal units for every vertical unit.  (This is obviously just an example, you may scale appropriately depending on altitude and duration of flight)
Descent - This is probably easiest to define/predict for “typical”, and there are a few different ways to figure out what a descent profile might look like:

Vertical speed:  For passenger comfort the flight might target 1500-2000 FPM down depending on airspeed and altitude.  At 250 knots, (speed limit below 10,000’) and 1500-2000 FPM rate of descent, the gradient would equate to 13-17 units horizontally for every unit vertically.
The Flight Management System may be programmed to calculate a default 3 degree descent for the autopilot to follow, and most approaches are about 3 degrees.  So, 3 degrees is probably a good descent angle to use for typical.
The Flight Management System on larger airliners typically calculates a descent based on a descent speed profile, which then results in a varying descent angle and vertical speed, just the climb section described above.
Finally, for rough planning pilots us a 3:1 rule of thumb for descents.  This means that for every 3 nautical miles you should plan to lose 1000 feet.  This equates to a ratio of about 18 horizontal units for every vertical unit.

I hope this helps.

@Bianfable, Thanks for that perspective.  My experience is limited, but in the Dash 8 Q-400 I am currently training in we either follow a VNAV path, or target VS.  If you would like to suggest an edit or convert your numbers into a descent gradient I would be happy to add the info to my answer...

@Bianfable, I tweaked my answer a little in the descent section, please let me know if you have any suggestions.

I added another bullet to the descent section to cover my points from the comments.


text:
Which airliners have carried the most passengers?
I'm fairly sure that the Boeing 737 series will be in first place, with the Airbus A320 family coming second and perhaps the 727 third.
However, perhaps there are some estimates of actual figures that would provide more information.
See also Which individual aircraft has carried the most passengers in its lifetime? for a related question about passengers carried by an aircraft series.

Do not forget the DC-3...

@xxavier More 737s are flying right now at this moment than DC-3s were ever built. I don't think the DC-3 will be high in the charts at all.

As you note the 737 and A320 have a huge leg up on most other aircraft simply by production numbers. The 737 is the unquestionably the most produced large passenger jet aircraft in history when compared to the A320 it has 20 years of extra flights under its belt which is a lot of time to move a lot of people. Its a bit dated but Boeing provides a bit of data on this and estimates:

• The 737 family has carried more than 16.8 billion passengers; that
  is equivalent to every single man, woman and child flying at least
  twice. (2013 world population was
  7.1 billion).*
*As of December, 31, 2013

Airbus provides a similar stat estimating:

15,6+ billion passengers carried since Entry Into Service
As of November 2018

So it seems that the 737 is clearly in the lead over the A320 as 5 years ago they had already carried an estimated Billion more passengers than the A320 has as of last month.
These are the only two large commercial jet aircraft that even appear on the most produced aircraft list linked above. Other sources quote the 747 as carrying 3.5 Billion passengers, the 727 comes in around 2.2 Billion in its lifetime and the 757 is somewhere in the 1.3 Billion passenger range which is similar to the A330's 1.3 Billion (as of 2013) none of which even approach double digits so the 737 is clearly the leader from the estimates I can find.


text:
Are aviation regulators generally slower to issue orders affecting their "own" aircraft manufacturers?
It did not fail to escape the attention of commentators that the US Federal Aviation Authority was amongst the last aviation regulators to issue an order grounding the 737 MAX.
Is there evidence to show that this is a pattern - that US, European, Russian (or other) regulators tend to be slower to issue orders or directives that concern aircraft made by manufacturers within their own country or bloc?
What would count as an answer
"Evidence" in this case would be based on figures showing time to respond by different agencies in multiple cases (enough to indicate a pattern).
What does not count as an answer
I am not asking for opinions on whether the FAA or EASA or other agencies are politically sensitive (I am sure they are).
Though I am willing to bet that it will turn out to be the case that agencies tend to be slower to act when it would affect interests that are closer to home, I don't need to read speculation about why; I don't think it will be any more illuminating than the interminable exchanges about how Airbuses are better than Boeings or vice-versa.
Similarly, although questions have been raised and concerns expressed about the FAA's inspection regime and the degree to which it allowed Boeing to self-certify aircraft, that (and the regimes and processes of other regulators) is not the question here.

I should have guessed that this would quickly descend into ra-ra declarations about superior American pilots and ways, backward and corrupt non-western political systems, and so on. I wouldn't want to deny anyone their fun, but it's exactly what is not an answer to the question.

American commercial aviation didn't become the safest activity known to mankind by reacting in knee-jerk fashion to incomplete accident investigations and hysteria.  Grounding the Max was 100% political, no more, no less.  There's a reason that China led the way on that... think geopolitics.  IMHO, what should have been grounded are airlines that allow 300-hour FO's and which never allow their pilots to gain enough experience hand-flying the aircraft that the "Maintain Aircraft Control" step of the QRH is beyond their ability when things go off the usual script.

@RalphJ Two crashes of brand new aircraft within a couple of months is not very safe.

@RalphJ -- I think the problem lurks deeper (i.e. "who forgot to flight test the runaway stabilizer checklist to make sure it'd actually work when the 300-hr FO tried to action it, instead of turning said 300-hr FO into a 300-hr test pilot because nobody's tried to use the checklist in the airplane before?")

This whole thing was a major breakdown of the internal system safety analysis, that validated an architecture with a single-point-of-failure, when the required redundancy could have been implemented by mostly software, motivated by the need to avoid any kind of additional training requirement to keep a common type rating, so they couldn't even brief pilots on the system and the potential failure modes.  Nobody in any flight deck knew the system was even present on the aircraft.  Plus the FAA signing off on same.  Although your point is valid @RalphJ, this is mostly a failure by Boeing.

@Koyovis  American commercial aviation. Not Indonesian, not Ethiopian. Part of that difference is pilots with the experience to maintain aircraft control (as in, not exceeding VNE in level flight with takeoff power set) even without the autopilot & with distractions.

@UnrecognizedFallingObject  The checklist works fine; the problem wasn't to 300 hour pilot, it was being so unimaginably far above VNE that the stab trim bound up. At normal (under control) speeds, it works fine - witness the Lion air flight prior to the crash where they ran the checklist & kept flying fine.

@RalphJ Have there been any incidents in the US with malfunctioning AoA vane on B737MAX, successfully negotiated by US pilots? It could be the ability of the pilots, although these same pilots did much better with the previous type B737. It could be the very superior ability of US pilots, therefore not having to take the same caution as the rest of the world. But before we know for sure, better safe than sorry. Two crashes of brand new aircraft with loss of all life requires prudence.

I was many years in the industry with an OEM and have had various minor peripheral roles in certification and continuing airworthiness projects at the system level, giving me a kind of ring side seat.  And I would say that you tend to get more hair-trigger reactions from non-western regulators (China/Russia et.al), and that this is because there is a way lower level of trust between the regulator and OEM than with Western regulators.
Now, although it all went to hell in this case with respect to Boeing/FAA, there has to be some level of trust or else the regulator would have to constantly audit the manufacturer every time they scratched their butts.  It doesn't mean that the FAA just lets Boeing do whatever it feels like, but it does mean that they will tend to accept reports and analysis submitted without too much cross examination if things seem reasonable, and that's how situations like this can slip through.
Unfortunately the trust relationship broke down in the MAX case because Boeing perhaps could be said to have abused its trust relationship with the FAA.  Having been burned, you can be sure the FAA will tighten up its oversight of Boeing quite a bit, for a while anyway.
In non western countries the trust relationship is not very robust, so regulators will pull the trigger quite quickly.  This can cause its own problems.  You also have the tendency of non-western regulators to have much more severe punishment regimes, to the point of jail time for mistakes made in good faith.  This encourages people to hide their mistakes (a major problem in both China and Russia in manufacturing, maintenance and flight operations). So in the long run, in spite of the MAX fiasco, the low trust regime is much worse.

Entertaining ("you tend to get more hair-trigger reactions from non-western regulators" etc) but categorically not an answer to the question. The question is about evidence of slower response times.

@DanieleProcida You are never going to find that answer, unless by some amazing miracle someone here has actually done detailed studies of different regulators and measured input-to-output periods and compared them, like comparing ambulance operators.  Ain't gonna happen, so forget it.

I hesitate to call it miraculous, but Wikipedia gives a timeline for 737 groundings by different operators, showing who acted sooner and who acted later. You don't need "input-to-output periods" to see that in this case, the FAA was slower to act than other regulators.

Your question is about the response times for regulators dealing with OEMs in their own countries.  How fast does Russia respond to a Russian OEM; how fast does Transport Canada respond to and Canadian OEM, that sort of thing.  This situation of regulators outside moving before the FAA is different and is more of an outlier.  In general, regulators follow the lead of the host country's regulator absent special circumstances.  And this one is very special.  As I said, the FAA put too much trust in Boeing and Boeing abused it.  But in the longer term scheme of things it's a relative outlier.


text:
How universal are air traffic control systems, training and practices?
A commercial pilot, especially in the airline industry, is in a role that is essentially the same as that of a pilot who is employed and was trained in just about any other part of the world.
Does this also hold for air traffic controllers? How similar will the training and practices and the systems that they use in their work be?
For example:

Are qualifications obtained in one country or region recognised in others?
How much additional training would be required for a controller who has trained and worked in one European Union country, in order to take a similar role in another? Or in a non-EU state?
Would the hardware and software used by a controller in one country be familiar to a controller from another?

Related: https://aviation.stackexchange.com/questions/64205/in-the-us-is-there-much-variation-among-the-various-artc-centers-as-to-where-a  .  Apparently ATC practices are not even completely standardized from one region to another in the US.

A part of the answer lies in the fact nearly all countries  follow ICAO recommendations. The details are in the annexes to the Chicago convention. While this doesn't cover the domestic traffic, all international airports and traffic control agencies follow the same principles. This ultimately rubs off to domestic airfields and traffic. For example ATC and aeronautical documentation are nearly standardized worldwide.

This question is analogous to asking, for example, how universal are law enforcement  systems, training and practices. At the highest levels you may find similarities driven by things like EU regulations, to pick one, but as you dive deeper into the implementation of those three things, you will find differences, from minor to wildly diverging. Police officers and air traffic controllers may receive very similar basic training, but once they move off from there and into the field, things can and will change dramatically.
The same explanation holds for commercial pilots. A pilot flying cargo in Alaska on Cessna 185's, for example, will be trained in and execute procedures and practices completely different than a commercial pilot flying Britten-Norman Islanders on puddle jumping routes in the Caribbean.
Bottom line is it is challenging to answer this question without context, i.e. two countries you wish to compare.


text:
May airlines cancel unprofitable flights?
If a given flight is unprofitable for an airline (occupancy ratio or – more precisely – revenue from ticket fares and cargo below the break even point) and if operating this flight incurred a bigger loss than cancelling it ...

May the airline cancel the flight ? (If no, why not?)
Will they ? (If they may but won't, why not?)
May/will they state the true reason or seize a pretext (e.g. technical defect) ?

Please distinguish between scheduled flights and charter flights.

It was common for American Airlines to cancel commuter flights from San Juan, PR to Mayaguez when there were only a couple of seats sold on the flight. I questioned the practice but never got a straight answer from the ticket sales people in San Juan or Mayaguez.

This may be a better fit on travel.SE, because it seems (to me) that the question is mainly about passengers and business practices, not aviation per se.

@Pondlife: What is the rationale for closing this question as off-topic but allowing Q16692 which asks about a different aspect of the exact same topic?? Honestly, the closing tendencies here appear frustratingly arbitrary to me and frankly seem to get out of hand.

Personally I think that question is off-topic too but in the end it's the community that decides by voting. This is really a topic for the meta site but we don't have have strict rules here, only guidelines, and the community decides how closely to follow them in each case. Sometimes that means things are inconsistent but that's perfectly fine, it's part of how StackExchange works. If you feel strongly that our on-topic guidelines should be clarified or even changed, you're always welcome to bring that up in meta.

Please read the Montreal Convention and then decide if you accepted a correct answer!

@PeterKämpf, the Montreal Convention applies to international flights between nations that honor this regulation. My answer so far does not cover international flights, that is true, but incomplete and incorrect is not the same. Would you mind to point  out which part of my answer is incorrect?

The question relates to regulations and operational constraints. That is on topic.
Only the discussion of passenger rights is off topic here.

@bogl: Then what about the Regulation (EC) No 1008/2008 of the European Parliament and of the Council of 24 September 2008 on common rules for the operation of air services in the Community? Why don't you read source material first? And if you now claim that this is not for the US: It is trivial to find the US equivalent.

@PeterKämpf, Sorry, but I am not interested in playing games with you. Please tell us which part of my answer is incorrect, or stop harassing me. Thank you!

Made similar experiences, but never expected to get a honest answer. After all, what is the fifth amendment for?

@PeterKämpf, please provide the correct answer!

Yes, airlines may cancel any flight. There is no obligation to execute a specific flight.
Their obligations towards the passengers depend on the local legislation. For instance:

International Flights: Airlines may cancel international flights, but passengers on flights between countries that honor the Montreal Convention are entitled to damage compensation if a cancellation cannot be justified.
European Union: According to Flight Compensation Regulation 261/2004,
 airlines are obliged to transport their passengers within a certain time frame. They will have to compensate passengers after a certain amount of delay is exceeded.
USA: There are no federal laws requiring airlines to provide passengers with money or other compensation when their flights are delayed. Airlines may book passengers on a flight of a later day. (Source)

Yes, airlines will cancel a flight if it is not profitable, but they will take into account all operational and commercial implications, and not just the profit on this particular flight. For instance the plane and/or the crew might be needed at the destination, or the plane has to park in a different place.
It depends on the airline if they will use a pretext. It is fraud if they use a pretext and avoid paying compensations to their customers.
I am convinced that most airlines are honest, but many
   are not.
In a charter flight all seats are booked through a travel company. The legal obligations of the airline should be the same.

I'd like to place some additional emphasis on 'the plane (or crew) is not needed at the destination', as there is a difference between what airlines sell (flights from A to B) and where there cost comes from (flight legs from A to A', from A'' to A''', and from A''' to C, for example). In isolation, a flight leg might be unprofitable, but as part of the larger network, cancelling that leg might bring more cost/trouble.

Ryanair apparently does this rather aggressively. Their reputation is for being cheap, and only for being cheap, so they don't have a reputation to lose when they cancel a flight purely for economic reasons. And, in fact, they can be so cheap, precisely because they try so aggressively to be profitable. It's really that simple: if the flight costs are higher than the compensation payout, cancel the flight. (Assuming the plane and crew are not required at the destination.) If the plane is required at the destination, calculate what it would cost to compensate those affected passengers, too.

Please read the Montreal Convention if you ever find the time to do so. This might motivate you to correct your answer.

Further to @Bram's point about "network effects":  the only time I've ever had an underbooked flight cancelled was a shuttle flight where a single plane & crew made a few flights back & forth between a hub and a small airport 3–4 times a day.  The airline just cancelled one of the out & back excursions, and rebooked all the passengers onto later flights that day.


text:
What prevents Low Cost Carriers (LCCs) from cutting cost that compromise safety?
I was reading an article about LCCs and here is an excerpt that got me thinking.

While budget airlines cut other corners to keep fares low, they do not
  — actually cannot — compromise the safety of their passengers in the
  process.

What prevents low-cost carriers from cutting corners that might compromise the safety of their passengers?

It's not clear that LLC are in fact as safe as major carriers.

Why can't low-cost carriers compromise for the safety of their passengers?

Because there are laws and regulations which keep passengers safe, and low-cost carriers are not exempt from those regulations.

Nitpicking perhaps, but neither laws nor regulations by themselves prevent anything. It's the overseeing organisations and their personel that prevent. And those may well look the other way or just delegate the overseeing to the overseen...

There are two main factors.
The law
One, as other answers have mentioned, is the law. There are strict regulations that cover everything in aviation, from the way people speak to the training they must have and the specifications and audit trails of almost every piece of equipment they use.
Now it's true that people, especially unscrupulous ones who are keen to save money and might be willing to take a few risks in order to do that, don't always obey they law or follow regulations to the letter. And after all, there are laws against murder with much more significant penalties than for (for example) flying with a bit less fuel than the regulations say you should, and yet people persist in murdering.
The culture
However, the other factor is the culture. A low-cost airline exists in the culture of aviation, not independently, and the culture puts safety first.
Each person working for the airline will have trained and may have worked outside the airline, and will already have acquired attitudes and ways of thinking that prioritise safety and adherence to regulation.
Each person working for the airline will be working with and encountering aviation industry individuals from outside the airline, on a daily basis - if they didn't share similar attitudes and ways of thinking, they'd stand out a mile, and so would a corporate culture that similarly failed to embrace the industry's culture of safety.

Beside all the regulations that require certain maintenance action to be done on aircraft in order for the aircraft to remain air worthy, the simple answer is if LCC compromise safety in order to cut cost no one will want to fly with them and they will lose business.

It's not necessarily true that no one would want to fly with them.  It's simply that they can't LEGALLY do so.  (Though I would not be surprised to find that they actually do cut corners on maintenance &c, and hide the fact.)

@jamesqf: I think it's a valid point that cutting corners on safety is bad business, though.

An airline will need to have an 'Air Operator Certificate' (AOC) or something similar issued by the state. In order to have such a certificate it has to show that its operations are in compliance with the national laws of the country. There is also an 'Operations Specification' which details the type of operations the airline can carry out.
The respective manuals will need to be checked by the authorities and the actual operation will be audited.
For operations into other countries, the other country may require you to undergo a 'foreign operator air license' which will require a check of the manuals and possibly an audit.
The list of checks is endless and usually in the long run it is easier to follow than try to cut costs related to safety and security.
The airline can have its AOC suspended for unsafe operations until they fix the operations. Other countries can ban certain countries from operating in their airspace if there are security concerns. The EU, for instance, maintains a blacklist of airlines banned from entering their airspace.

Along with being illegal and potentially looking bad, generally, people's lives are in the balance but historically this has not always stopped the more unscrupulous airlines in the past.
Arguably Alaska Airlines cut corners that ultimately lead to the fatal crash and deaths of 58 people on flight 261. From the NTSB report:

Alaska Airlines' maintenance and inspection of its horizontal
  stabilizer activation system was poorly conceived and woefully
  executed. The failure was compounded by poor oversight...Had any of
  the managers, mechanics, inspectors, supervisors or FAA overseers
  whose job it was to protect this mechanism done their job
  conscientiously, this accident cannot happen

Cutting corners on training, improper cargo loading and generally poor handling of hazardous materials also lead to the fatal crash of ValuJet Flight 592 killing 110 people. There is some interesting info on it in this paper from the FAA

There has been push from various low cost carriers to introduce what is effectively standing room only aircraft or sections of aircraft for an even lower fare. However this does not pass the seating requirements that the FAA/EASA generally have so no one has yet to actually bring it to market. It will be interesting to see if they can ever push the regulation through.

While not technically an answer: I just want to point out that (most) LCCs do not have the financial means and public image credit to survive even a single minor incident or crash.
A simple runway overrun or engine fire might be enough to literally bankrupt the company within days, and (most) LCC managers are painfully aware of that. The incentive is therefore quite high to avoid any incidents and keep safety levels way above the minimum required by law.


text:
What would be the power requirements and performance of a Scroggs-type delta-wing aircraft with a 250 cm span?
Roy Scroggs, a tailor, patented in 1917, US1250033, and in 1932 US1848578, a Delta Wing plan form machine 'Flying Dart', flight tested

  with an OX-5 engine, they say anything would fit an OX-5 powered airplane, as in Besler Steam Airplane, it had same Leading Edge swept, 60º, as Handley Page HP 115 of 1961, that tested the Concorde wing (

 )
The page of an study from the French ONERA about vortex over a Delta plan wing is an experiment conducted with the type of pointed wing in the description by Jones, and in the Langley tests on A Lippisch DM-1, but it's not sure if flow will be identical on a truncated point Delta, as used in first Saab 'Draken'.??

I wonder what the power requirements and performance would be of an Ultralight Airplane with a wing span of 250 cm, maximum width allowed on the road, and a total weight less than 299 kg. 115 kg empty weight is the class limit for Ultralight

First, thank you for unearthing this unorthodox design!
However, it shows that paper airplanes don't scale well. Nevertheless, it follows the same laws of physics, so the same equations should apply for a quick performance evaluation. What have we got?

A flat plate airfoil. Maximum lift coefficient will be between 0.7 and 0.8, so minimum speed at a given wing loading will be quite high.
A sweptback delta wing of low aspect ratio, so the lift curve slope will be low. For the same lift coefficient it needs much more angle of attack than a regular airplane. Since upper side flow is separated, airfoil drag will grow with the square of the angle of attack, and the low lift curve slope ensures that drag will be needlessly high.
A big fuselage with a lot of wetted surface area.
A massive separation area at the back which will produce a lot of drag without any benefit, except for improved directional stability.

Let's start with induced drag. The low aspect ratio ensures a nearly elliptic lift distribution, and lifting 300 kg with 2.5 m of wing will create $$D_i = \frac{2\cdot(m\cdot g)^2}{\rho\cdot v^2\cdot\pi\cdot b^2} = \frac{180000\cdot g^2}{1.225\cdot v^2\cdot3.14159\cdot 6.25} = \left(\frac{g}{v}\right)^2\cdot 7483\;N$$
At 20 m/s flight speed this will equal 3,600 N of drag and at 40 m/s flight speed a quarter of that (900 N). Multiply by speed to get the power needed to overcome just induced drag: 36 kW at 20 m/s and 18 kW at 40 m/s.
But what speed is achievable? We need to guess the wing area in order to find the stall speed. I assume 4 m chord and 1 m span at the leading edge, so the wing area is 7 m². Using a maximum lift coefficient of 0.75, this means you cannot fly slower than 30.24 m/s. Whoa! That is 58.8 kts or just below the 61 kts limit for the old FAR 23.
Let's add some safety margin and continue with 40 m/s flight speed, which corresponds to a lift coefficient of 0.43. Now to the angle of attack - this requires to know the lift curve slope. As a slender body it will be slightly below $\pi\cdot\frac{AR}{2}$, and with an aspect ratio $AR$ of below 1 this is maybe 2.2 per radian or 0.04 per degree. At $c_L$ = 0.43 this is 11°. Multiply the weight with the sine of 11° and you get the cost for flying a fully separated top wing: 506 N. Dragging this through the air at 40 m/s needs another 20.2 kW.
Now to the wetted surface: The vertical surfaces add maybe another 4 - 5 m² to the 14 m² of the wing. The Reynolds number is 11$\cdot 10^6$, so the friction drag coefficient is as low as 0.003 if the surface is smooth. Since the upper wing shows separated flow, I will continue with 15 m² surface area. At 40 m/s this equates to 44.1 N of drag, needing 1.76 kw of power.
The fixed and unfaired landing gear will be another source of drag; here I simply double the friction drag in order to account for it, too. I expect that a detailed drag analysis will not change the result substantially.
Your idea of filling the separation area with a ducted fan is probably costing even more performance. Please consider that now you have a high-speed stream of air flowing through a duct running along the length of the fuselage. Better to narrow the fuselage in the rear half so it ends in a vertical rudder.
If we now assume that you avoid any boattail drag this way, flying straight and level at 40 m/s will need a total of 42 kW. Add 50% for manoeuvring and climbing (those extra 21 kW translate to a climb rate of more than 6 m/s - looks OK) and a propeller efficiency of maybe 75% (look at this answer - that could be as low as 60% in reality!), and you end up with an engine requirement of 84 kW or 112.6 HP. It looks like you need a Rotax 914 or similar engine, the mass of which will eat up one third of your budget.

Thanks for your excellent and kind help. You can watch in YouTube video that the Scroggs, man carrying machine, actually went airborne, 3 mt above ground, it's described in 'Aerofiles'. Goal is 255 cm span, 2 meters and a half, the maximum width allowed for a road vehicle, you know main issue in 'Flying car' is passive road safety rules; SAE book: 'Vehicle compatibility in Automobile crashes', depicts situation. Idea is making first an 80 cm span Aeromodel, with a ducted fan inside, perhaps with an Space Shuttle Wing Plan. 299 kg come from me, 135 kg, plus engine, structure, fuel,...

Instead of the open, drag generating end in the Scroggs machine, a Ducted Fan blowing rearwards inside fuselage, the engine cooling and exhaust heat added to increase thrust will reduce problem stated in point 4. The wingtips acting as ailerons,as in Hoffman design, see 'Wingless wonders', YouTube,  elevons in the rear straight trailing side of wing, plus a vertical Fin-Rudder upside providing control, even if I'd like an 'Inverted Vee', or 'A' type of 'tail unit, inducing roll in the right sense at turns.

@Urquiola  thanks for showing this design.  Inspired by record setting "Paper Airplane Guy" and the 225 foot indoor throw, the 60 degree HP 115 was my first choice for scratch builds too!  However, deltas do not provide nearly as much lift as straight wings, hence 112 HP requirement!  Notice the Aeronca C3 flew quite well with 36 HP at around 1000 lbs.  You may wish to check out high wing or bipe ultra lights.

Thanks. Your comments about in-Fuselage Ducted Fan propulsion seem confirmed in the Stipa-Caproni results. I attach documents about a similar US design, and DF efficicay figures from the Hovey booklet, not that bad. They were preparing an electric DF version of a MiG fighter in Russia. Salut +

With the Space Shuttle Wing Plan and the fixed span (regulatory requirement) of 2.5 meters, the overall Wing length would be 6,1 meters; a Wing Area around 7 m 2. What about a Wankel RCE? Please see www.rotaryeng.net

@Urquiola you have some very good thoughts and info.  Electric motors ducted fans work very well with R/C aircraft, and, with their high rpm, can get you into fan blade design.  They are the "poor mans" jet.  Have heard about Wankels greatly reducing vibration (very important in aircraft).  Might be good as a next question, I'll look for efficiency (miles per gallon) vs the opposing piston 4 cylinder, the V12, and the radial!

You may like: 'Base Drag and Thick Trailing Edge', Sighard F. Hoerner; 'Flight Tests of External Modifications Used to Reduce Blunt Base Drag', NASA TM-100433, S Goecke Powers; patents: US2008737, 'Shuffled Wing Airplane', H Wirth; 'Airplane', PA Richard, US2241521; 'Hollow Fuselage Airplane', A Schramm, US2071221; 'Stipa Monoplane with Venturi Fuselage', NACA TM 753, 1934.

The twin Electric Ducted Fan MiG isn't an AeroModel, but a Man Carrying prototype; about Electric Ducted Fans, you may like the 'Lilium Air Taxi'  https://youtu.be/uGrAwc-cbrY

Name of Ducted-Fan MiG mini replica is PJ-II Dreamer, it appears in the site: minijets.org also in YouTube. Patent GB404166, by Fernando Gallego-Herrera, is about a tailless, Venturi Fuselage machine, it all variant responses to the question

A comment signed: 'Rick the Rocketeer', indicated Space Shuttle Airfoil is NACA 0100-64 at root, plus a derivative of NACA 0012, 10% thickness at root, 12% thickness at tip? The Stipa Hollow Ducted Fuselage NACA tests pointed increased drag, but I can't understand the reason why duct has the shape of an airfoil (See Modern Mechanix, March 1933, cover&page 77). Why not a constant diameter duct? There's an interesting Naval Postgraduate School, CA, Thesis by William H Reuter, 1993: Space Shuttle -with added Canard foreplane- at Mach 5.8 and 50º Angle of Attack. Fine prospects!


text:
Do the operations practices of low-cost airlines represent an increased safety risk?
Low-cost airlines spend less money on their operations. In most (not all, clearly) contexts of life and business, more money spent on the same kind of thing means a higher quality product or service.
Do any of the cost-saving measures adopted by low-cost airlines lower the quality of their operations (flight operations, aircraft maintenance, crew training, crew quality) in a way that could affect safety?

Following this reasoning, Concorde would have been and would still be the most safe aircraft.

You may find this question about airplane safety (not airline safety) on Travel Stack Exchange to be helpful : https://travel.stackexchange.com/questions/28978/which-airplane-is-the-safest

Edited to remove the unnecessary reference to particular airlines. In its essence this is a perfectly reasonable question. If the answer is an emphatic no, that doesn't make it a bad question.

The question is intensely opinion based. You can have low-cost airlines who take risks with safety, while other low-cost do not. What makes an airline low-cost is the cost structure. Airlines must maintain aircraft to airworthy standards no matter if they own the plane or if they lease it.  Comparing low-cost and traditional carriers safety performance is difficult since the airline safety data is not public information.

We should not make statements about any particular airline. Air travel is and remains very safe, due to:

Good practices imposed by the regulatory institutions.
Feedback of things that went wrong in the past to prevent them from happening in the future.
Very mature engineering and technology implementation
Lots and lots of experience with reliability numbers over the decades.

That being said, proper procedures do need to be followed and it could theoretically be the case that only minimal maintenance is carried out at some airlines, while others are more pro-active and lead the field in safety practices. Think Rainman, and the airline that has never crashed.
Safety numbers of all airlines in the developed world are very good. Cheaper airlines must follow the same practices as more expensive airlines. The cheaper tickets are the result of better business practices, not of worse maintenance practices.

Edit
@Michael Kjörling points out a TV episode about a case of Madrid airport being closed, all traffic lining up for Valencia, and three RyanAir aircraft declare fuel emergency. Plus a LAN Chile declares engine out Mayday.
Don't know if this counts as evidence though, TV programs must find or produce a topic that is sensational enough for transmission. The safety numbers are still on RyanAirs side, their safety record is pretty good.

KRO - Mayday! Mayday! would seem to disagree with you, at least to some extent. I'm not in a position to judge the accuracy of the claims made therein, but it's probably undisputable that even if everyone is technically in accordance with the same regulations, there are differences in terms of the available margin for error. However, I absolutely do agree with your statement that (particularly commercial) air travel is and remains very safe.

"We are riding on the edge of regulations. We are legal, but we are riding on the edge" is what they're saying at 32 minutes in the video. Yes another airline may put more fuel on board and have to charge more, and then passengers don't buy their tickets. It is always the consumer who takes the final decision. It is the regulatory institutions who police safety regulations. And is the airline is within the legal limits, that is all that can be asked in a situation where all of us have proven that we do not wish to spend any money on additional safety.

"And is the airline is within the legal limits, that is all that can be asked in a situation where[...]" I agree, and like I said, I'm not in a position to judge the accuracy of the claims made. The legal limits are there for a reason, obviously. Bottom line for me is math; if Airline A sells 200 tickets at an average of €40 each, and Airline B sells 150 tickets at an average of €100 each, and flies the same type aircraft the same route, then Airline A must be cutting costs (compared to Airline B) somewhere.

One can hope it's just the profit margin, but that is probably overly naiive. In all fairness, low-cost carriers typically fly to airports that are farther from the major cities, so it's not the exact same route, but I'd expect them to be largely similar enough that from a flight planning POV, there is little difference. (E.g. landing fees are a potentially very different matter.)

@MichaelKjörling The cheaper airline could be 'cutting costs' by charging extra for things like seat assignments, checked baggage, meals on board etc. that the second airline includes in their ticket price.

@Pondlife Certainly.

1. Low-cost-airline does not mean the airline has spent less on aircraft, or skipped on maintenance.
In fact, at least in Europe, some of the best maintenance available in the industry is by the low-cost carriers. I’m  not a business analyst so I will try to explain this a bit more simplified:
The reason you get low ticket prices is because low cost carriers fly (relatively) brand new aircraft, with a very low operational cost. Meaning, per hour of flight, the airplane is cheaper to operate than the older not-so-economic ones. These new aircraft are more pricey though, so you end up paying a lot on the montly lease. What you end up with is a fleet of aircraft that cost a lot of money (montly  lease) to own, but pretty cheap to operate. And, mind you, you have to pay the lease whether the plane flies or stays on ground for maintenance, so the name of the game becomes: keep the planes flying as much as you can.
Compare this to a flag carrier that owns all of its aircraft. If a plane stays on the ground, it is not making money, but it is not costing anything either.
2. Paying for good (preventive) maintenance ends up being a lot cheaper for most low cost carriers.

A low-cost airline has very slim margins and nothing eats up that margin faster than a delay. Eg a LCC charges a cheap fare but pays the same airport fees, pays their staff similar (or more) to a full-service carrier (but works them harder). In the event of a delay they may try to get away with a cheaper meal and their hotel may not be a 3-star but even these cheaper alternatives will make that flight a 'loss'.
Also LCCs tend to fly their aircraft more hours in a day with minimal ground time between sectors so the aircraft needs to be in good condition. While scheduling does try to have some 'firebreaks'in the days schedule to recover some unscheduled down time it does catch them out occasionally.


text:
Why a higher compressor pressure ratio results in higher Thrust and lower TSFC?
My professor usually asks us in our exams to plot the Thrust-Pressure ratio and TSFC - Pressure Ratio charts and justify the tendencies observed.
(TSFC = Thrust Specific Fuel Consumption).
We all know that, as the compressor pressure ratio ($\pi_c$) increases, the higher the Thrust ($F$) and the lower the $TSFC$. Here is a graph (which was taken from the book Fundamentals of Jet Propulsion with Applications, by Ronald D. Flack).

This chart was made after taking the final expressions for $F$ and the $TSFC$ (which are huge) and plotting them against $\pi_c$ using a computer. Obviously we don't have computers in the exam, nor we have enough time to derive such complex formulas. Here is an example for the (non-dimensionalized) $TSFC$:

Instead, I want to impress him with a convincing, non-mathematical, qualitative explanation of this behaviour.
Therefore my question reduces to: why, physically speaking, does the thrust increase with $\pi_c$? (The answer to "why does the TSFC decrease with $\pi_c$ can be answered saying that $F$ and $TSFC$ behave inversely to one another).
This is my attempt:
As the compressor pressure ratio increases, the flow will be mixed and burned more efficiently in the combustion chamber, therefore increasing the energy available for the nozzle to accelerate the flow. Since the jet thrust increases with the exhaust speed, we can rest assured that a higher pressure ratio will translate into a higher thrust.
The $(TSFC-\pi_c)$ behaviour will be inversely proportional to $(F-\pi_c)$, since $TSFC=\dot{m}_f/F$
NOTE: My explanation of the process, I think is a bit incomplete. For example, it doesn't explain why there is a $\pi_c$ for which $F$ will actually start decreasing (in the graph, this $\pi_c$ value is around 15).

Because the constant pressure lines are diverging with increasing enthalpy.
Have a look at the right most diagram.
The two lines are constant pressure lines, with the higher line indicating the higher pressure, and as you can see, they are diverging.
1 is the beginning of the cycle, thus at ambient temperature
From 1 to 2, we increase the temperature, and go to a higher pressure in the compressor. This will cost some energy, which is delivered by the turbine.
From 2 to 3 we increase the Temperature at the the same pressure by adding energy.
From 3 to 4, we decrease the temperature and pressure in the turbine (and use some of the extracted energy to drive the turbine)
From 4 to 1, we use the energy in the flow to propel us, and we go back to ambient temperature.
In principle we could also skip the higher pressure, and simply go from 1 to 4 and back to 1 (which is just heating air, and propelling yourself by the heat.)
But by moving to a higher pressure, we can take advantage of the divergent pressure lines. This yields us a benefit because the distance 3-4 is larger than 2-1. This means that if we use the turbine to drive the compressor, we gain some 'free' energy.

Source
Following OSUZorba's comment:
To illustrate the point of the 'free energy', look at the image below, here two extra stations are added the inlet, which isn't that important for now, and the nozzle. Note that the vertical distances of the compressor and the turbine are equal (they have to be, because one drives the other), thus
$$ \begin{aligned}P_{comp}  = P_{turb}  &\Rightarrow \Delta T_{comp} = \Delta T_{turb} \\ &\Rightarrow (T_3 - T_2) = (T_4 - T_5) \end{aligned}$$
But because of the divergent nature of the lines, we have some extra energy left. We use the nozzle to optimally expand the high pressure, high temperature flow and extract the 'extra' energy.

Source

So, the higher the pressure $p_2-p_1$, the higher $h_3-h_4$. Am I right here? This means that the turbine is working more efficiently, because it is able to extract more energy from the incoming flow. But I can't see a direct relationship with the thrust. Can you help here? Thanks for your awesome answer.

Oh, and by the way, would you be able to explain why there is a point at which further increasing the pressure won't increase the thrust, but actually decrease it?

I think, if you further increase the pressure, you might run into other issues. The general limiting factor is point 3, there is only so much temperature the metal in your combustion chamber (and the following turbine) can handle. Another issue might be the compressor itself, it might not be able to reach extremely high pressures with flow separation at the blades.

I'm not sure, but I think that the pressure lines are getting closer and closer to eachother with higher pressures: see here

Good points @ROIMaison. I'm still trying to find a correlation between $h_3-h_4$ and the thrust. I mean, the higher the $p_2-p_1$, the higher the temperature difference between 3 and 4 (since as you said, constant-pressure lines are divergent). This means more energy available in 3, which I guess this translates into more kinetic energy for the nozzle to convert to pressure energy. Am I right?

Yes, basically, you try to get as much energy (heat and pressure) in the air, and then let it release the energy, propelling you forward

I didn't understand what you said: " I'm not sure, but I think that the pressure lines are getting closer and closer to eachother with higher pressures". Am I right assuming that as $p_2-p_1$ increases, so will $h3-h4$ but more quickly?

Let us continue this discussion in chat.

This is a very good response, but in a non-turboshaft engine there is another point between 3 and 4, which is the exit of the turbine and entrance of the nozzle. So as the pressure ratio increases less of the energy in the turbine is used to drive the compressor and more is available for thrust

@OSUZorba, you're right, I was looking for a good picture of a T-S diagram of a jet engine, but I couldn't find any with all the stations and nice constant pressure lines (to illustrate my point), if you know any, feel free to add them.

Now I understand everything clearly!! Thank you ROIMaison. Also thanks to @OSUZorba for chiming in and adding extra value to the answer.

For some reason it won't let me edit my comment. Just to clarify, the turbine doesn't take less total energy to run a higher pressure ratio compressor, but a lower percentage of the total energy available to the turbine.

@ROIMaison This NASA page has a good TS diagram in it: https://www.grc.nasa.gov/www/k-12/airplane/brayton.html


text:
Why fewer operations on weekends?
Why are there fewer air carrier operations on weekends? Looking at PHX data, we have the following annual trend that clearly shows a weekly drop of about 8% on weekends:

and when I average the data a strong weekly pattern is evident:

Is this simply telling us that most traffic is business related? Or is it, in part, that major cargo haulers don't work as much on weekends? As the charts show, we're talking about a 1200 vs 1000 daily operations difference.

For whatever it's worth, this pattern of fewer operations on Saturday and somewhat fewer on Sunday that you observed at PHX also appears to hold at almost every major U.S. passenger airport. I'd assume that the lack of business travelers is indeed the reason, but I don't have any hard data off hand to back that up.

I agree and just reached the same conclusion. I posted my own answer based on studying a few different airport types. Thanks for your input.

Your data TELLS us that the airlines schedule fewer flights on the weekends, from which we can INFER that there is less demand for travel on those days.  (The numbers of cargo flights at PHX are dwarfed by the numbers of passenger flights there. The data at a major cargo hub like MEM or SDF might be different in that regard.)
To what extent that is driven by business travelers or leisure travel patterns, is beyond your data, and might even get into some fairly complex data and calculations that USAir and Southwest and others use to build their schedules.

I realize the data doesn't tell us this, but I was wondering if there was some   well known - behavioral - factor that the airlines were responding to other than simply reduce supply to match reduced demand.

and of course, thanks for the reply!

The black box that is airline scheduling :)

Businesses work 5 days a week. Non-business travel happens 7 days a week.
Other factors kind of even out: Business travel can still happen on the weekend, but ticket prices are normally cheaper mid-week which attracts non-business travelers.

Additionally, businesses are willing to pay higher prices for their travel needs, as opposed to leisure travelers who tend to shop around for the lowest prices.

After further study, I believe the answer is that the weekday vs weekend effect is caused by business travel. The effect is present at major hubs and weakly present at locations that are either strongly recreational, or are not obviously major business destinations.
For example, Aspen shows a fairly uniform day of week distribution:

while the annual data shows obvious trends associated with winter and summer recreational activities.
Also, at smaller airports, it might be difficult for airlines to modify the number of flights that support business travelers.

The Aspen data shows highest traffic on Fridays and Sundays, probably because people come in on a Friday to spend the weekend and leave on Sunday.

Indeed and this effect is smaller than for business travelers. I think non-business travelers have more flexibility and will travel on weekends and/or want to take advantage of weekends, hence their day of week travel is more uniform across all days.